1. Field of the Invention
The invention relates to a glazing for solar protection, coated with a thin layer having a titanium carbide base. It is intended to equip automobiles.
2. Background of the Related Art
Many uses for glazings which reduce the transmission of solar radiation are known. Their purpose is limiting the energy entering rooms or vehicles to avoid an excessive heating and/or to limit the consumption of energy necessary for air conditioning.
Two techniques are known for reaching this goal, either the use of glasses tinted in mass by the addition of various metal oxides to the glass composition, or the deposit of thin absorbent or reflective layers on the surface of the glass sheets.
It is necessary to use a layer absorbent enough to absorb the solar radiation, and non-reflective enough to avoid discomfort and to remain in accordance with the relevant government regulations--particularly in the case of automobile glazings. Moreover, it is necessary that this layer to be resistant enough to chemical or mechanical attacks to be able to be used alone without being protected (as would be the case inside a laminated or multiple glazing).
For automobiles, there exists a demand for glazings of small thickness, but very heat absorbent. In modern automobiles, the glazed surface area has become very great and, further, windshields and rear windows are more and more inclined. This can produce a superheating of passenger spaces when they are exposed to the sun and increased passenger discomfort. That is why, to improve passenger comfort without reducing the visibility necessary for the driver, it has been considered to provide the different glazings of a vehicle with glasses having different heat transmission characteristics. For example, the windshield is the most transparent, the front side glazings are less transparent, yet less transparent the rear side glazings, and finally the rear window are the least transparent. Thus the requirement for maximum visibility through the windshield is met with light transmission greater than 75% according to the regulations but the other glazings for which the transmission requirement is less restrictive have greater heat absorption ability. Further, for reasons of weight, it is desired to use glasses whose thickness is the smallest possible. There is therefore a need in the case of glass tinted in mass, for a very dense coloring and for different degrees of coloring (four in the example chosen above). These two requirements are very difficult to meet in practice. A glass tinted for strong absorption of solar radiation is, as a corollary, derived from a glass mass itself very absorbent for the radiation of flames intended to heat the mass in the glass melting furnace. This characteristic very appreciably modifies the properties of the molten glass and particularly the convection currents in the molten glass, which is decisive in the production of a glass of good quality. That is why the density of the coloring of the glass generally is limited to fairly small values.
On the other hand, multiple intrinsic absorption coefficients, and therefore multiple colors of glass obtained thanks to additions of coloring agents in the melting furnace, which would be necessary to make the absorption characteristics vary without variations in thickness, would be applicable industrially only if the amounts of coloring agents consumed justified it: that is not generally the case. The traditional method which consists of changing the optical characteristics by acting on the glass thickness is not desirable because, for reasons of weight, it is desired to have a thickness of glass--generally tempered, except for the windshield--which is the thinnest possible, the limit being imposed for reasons of mechanical strength.
Further, the storage drawbacks that significant stocks cause should not be very different from what they are in the case of glasses of different compositions. The use of absorbent thin layers, optionally deposited on a glass that is absorbent, makes it possible to stock limited amounts of base glass, the various tints being applied on the product only in a final phase of production.
The techniques for depositing thin layers under vacuum using the cathode sputtering are well known. In particular, those which are performed in the presence of a magnetic field which multiplies the impacts of the ions on the target and accelerates the deposit are known. For example, German patent publication DE 24 63 431 C2 discloses such a process using a plane target and U.S. Pat. No. 4 116 806 uses a target in the form of a belt.
Likewise, techniques of reactive cathode sputtering are known which make it possible to obtain a thin layer by making the material of the target react with the gases of the plasma. U.S. Pat. No. 3 907 660 presents such a method for the deposit of metal oxide on glass.
Of the absorbent thin layers able to be used bare, i.e. without protection, on a glazing, nitrides, carbides or silicides of the metals of Groups IV, V, or VI of the periodic table are known. French patent publication FR 2 104 813 presents examples of nitrides of chromium, of molybdenum, chromium carbides, mixed carbides of chromium and titanium and various silicides. These layers are deposited by various methods such as cathode sputtering at radio frequency from targets consisting of the same material or, in the case of nitrides, reactive cathode sputtering from a target made of the metal concerned.
These layers act on the solar radiation both by absorption and by reflection, but their utility as layers intended for an automobile glazings resides in their relatively small reflection. However, to be used in an automobile it is necessary that the resistance to scratches of these layers be great, because any scratch will be visible both in reflected light, for example when an automobile is observed from the outside, and in transmitted light when from inside the automobile the countryside is observed through a side window, for example. Actually, if a very absorbent layer suffers a scratch, at the side of the scratch, its transmission is greatly increased, possibly reaching locally that of the glass without layer, which creates an intolerable appearance defect. This requires a layer particularly resistant to abrasion.